Nuclear magnetic resonance (NMR) spectrometers typically include a superconducting magnet for generating a static magnetic field B0, and an NMR probe including one or more special-purpose radio-frequency (RF) coils for generating a time-varying magnetic field B1 perpendicular to the field B0, and for detecting the response of a sample to the applied magnetic fields. Each RF coil and associated circuitry can resonate at the Larmor frequency of a nucleus of interest present in the sample. Nuclei of interest analyzed in common NMR applications include 1H (proton), 13C (carbon), and 15N (nitrogen). The RF coils are typically provided as part of an NMR probe, and are used to analyze samples situated in sample tubes or flow cells. The direction of the static magnetic field B0 is commonly denoted as the z-axis or longitudinal direction, while the plane perpendicular to the z-axis is commonly termed the x-y or transverse direction.
Several types of RF coils have been used in NMR systems. In particular, many NMR systems include transverse-field RF coils, which generate an RF magnetic field oriented along the x-y plane. Transverse-field coils include saddle-shaped coils and birdcage coils. Birdcage coils typically include two transverse rings, and a relatively large number of vertical rungs connecting the rings. Birdcage coils are multiply-resonant structures in which specified phase-relationships are established for current flowing along multiple vertical rungs. Saddle-shaped coils normally have the current path defined by a conductor pattern around the coil windows. A particular type of saddle-shaped coil design is the Alderman-Grant coil design. An original Alderman-Grant coil design having two vertical rungs and chip capacitors was described by Alderman and Grant in their paper entitled “An Efficient Decoupler Coil Design which Reduces Heating in Conductive Samples in Superconducting Spectrometers,” J. Magnetic Resonance 36:447–451 (1979). Other Alderman-Grant coil designs can have vertical slots defined in the vertical rungs, and can employ distributed capacitance structures rather than discrete chip capacitors.
An NMR frequency of interest is determined by the nucleus of interest and the strength of the applied static magnetic field B0. In order to maximize the accuracy of NMR measurements, the resonant frequency of the excitation/detection circuitry is set to be equal to the frequency of interest. The resonant frequency of the excitation/detection circuitry varies as
                    v        =                              1            /            2                    ⁢                                          ⁢          π          ⁢                                    L              ⁢                                                          ⁢              C                                                          [        1        ]            where L and C are the effective inductance and capacitance, respectively, of the excitation/detection circuitry.
Generating high-resolution NMR spectra is facilitated by employing a temporally and spatially-homogeneous static magnetic field. The strength of the static magnetic field can vary over time due to temperature fluctuations or movement of neighboring metallic objects, among others. Spatial variations in the static magnetic field can be created by variations in sample tube or sample properties, the presence of neighboring materials, or by the magnet's design. Minor spatial inhomogeneities in the static magnetic field are ordinarily corrected using a set of shim coils, which generate a small magnetic field which opposes and cancels inhomogeneities in the applied static magnetic field. Temporal variations in the static magnetic field are commonly corrected using a field lock. Field lock circuitry monitors the resonance frequency of a reference (e.g. deuterium) signal, and adjusts the static magnetic field strength to keep the reference signal frequency constant. Deuterium is commonly added to sample solvents to provide the field lock reference signal.
In general, the field lock reference signal and the NMR measurement signal have different resonance frequencies. Consequently, if the same RF coil is used to acquire both the field lock and sample NMR signals, a conventional RF coil optimized for the sample resonance of interest may not be ideally suited for the field lock reference signals. In some NMR systems, a single coil may also be used to perform NMR measurements for multiple nuclei of interest. In such systems, the coil may not be ideally suited for all resonance frequencies of interest. Improving the performance of NMR systems over relatively broad tuning ranges would be useful for enhancing field lock accuracy, as well as improving single-coil, multi-nucleus NMR measurements.